I was trading units between my armies and my city, but it said my city had 0/89 population while I was on the world map. I was scared so went inside and everyone was there 89/89. I left and tried adding my entire archer army to the city, 50 people strong, and it seemed to work. Went into the city and suddenly it was 139/89. I then added 10 more unit from a separate army but on the world map it again showed 0/89.

This occurred directly after siege of an orc city. I made it my vassal, transferred some units to that city, and that is when my main city no longer showed any troops on the world map. All troops i added to the city would automatically be garrisoned also. All 50 archers instantly had the garrison option set. I logged off and after a few hours came back and my city on the world map did show 149/89, however this bug could be exploited to create massively unbeatable defenses for people.